The key to jar salads is: it's all about how you pack your jar. If the dressing is the first layer and put crisp veggies, nuts and seeds directly on top of it, you create a natural barrier between the lettuce and the dressing until you shake it up when you're ready to eat. Serves 1 and makes 1 jar (1 cup of dressing)
To make the dressing, whisk together all the ingredients in a small bowl, or place them all in a small jar and shake very well. This dressing will keep in an airtight jar (minus the poppy seeds) in the fridge for up 3 days. This makes plenty of dressing for multiple salads.
Wash and pat dry the spinach and tear it gently into bite-sized pieces.
Pour 2-3 Tbsp of the dressing into a 2-cup Mason jar. Place the walnuts, then the cranberries and sunflower seeds, then the carrots, and the poppy seeds. Put the lid on tight.
When you're ready to eat, shake the jar really well so the dressing coats the leaves. Open and enjoy.